Multilayer infrared filter

ABSTRACT

An infrared filter that is formed as a laminated structure including a plurality of layers having substantially uniform thicknesses. The various embodiments include first layers that are relatively thick which are interleaved with second layers that are relatively thin as compared to the first layers. The second layers are generally one quarter wavelength optical thickness at a particular tuned wavelength, whereas the first layers are generally a plurality of quarter wavelength optical thicknesses. Methods for manufacturing the filters are described, including grinding and polishing procedures for obtaining the desired uniformity of thicknesses of said layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to infrared filters, and moreparticularly to infrared filters having a laminated, layered structuresand a method for the manufacture thereof.

2. Brief Description of the Prior Art

Far infrared filters have been previously made by vacuum deposition ofthick films upon suitable substrates. However, the individual films aretypically so thick that intrinsic mechanical stresses cause manyfailures. Such filters may take three weeks to produce and have only a30% chance of producing a successful product. Therefore they areexpensive, typically costing twenty thousand dollars for a single oneinch diameter filter at a wavelength of approximately twenty-fivemicrons.

U.S. patent application 3,551,017 entitled "Far Infrared TransmissionType Interference Filter", issued Dec. 29, 1970 to Toshikatsu Iwasaki,et al., discloses an interference filter having a plurality of layerswhich are arranged into two sets of layers, (F.A.L.) and (S.A.L.). Eachof the layers in the F.A.L. set has an identical optical thickness, andeach of the layers in the second set has an identical optical thickness,although the optical thickness of the second set of layers differs fromthe optical thickness of the first set of layers.

Other prior art known to the inventor includes U.S. Pat. No. 2,660,925,Light Reflector Which Transmits Infrared Rays, issued Dec. 1, 1953 to A.F. Turner which discloses a reflector of e.g. glass coated withgermanium, which transmits a relatively large proportion of incidentinfrared light. U.S. Pat. No. 3,000,375, Semi-conductor Heat AbsorptionMeans, issued Sep. 19, 1961 to M. J. E. Golay which discloses aninfrared filter comprising a semiconductor such as germanium or siliconon a base of rock salt. U.S. Pat. No. 3,188,513, Optical Filters AndLamps Embodying The Same, issued Jun. 8, 1965 to R. L. Hansler whichdiscloses an infrared filter having a film of e.g. silicon or germaniumcoated with a dielectric film. U.S. Pat. No. 3,331,941, Infrared Heater,issued Jul. 18, 1967 to J. W. Edwards et al. which discloses an infraredfilter comprising alternate layers of germanium and cryolite on a quartzsubstrate. U.S. Pat. No. 3,718,533, Composite Sheets For AgriculturalUse, issued Feb. 27, 1973 to S. Shibata which discloses a filter foragricultural use employing layers of polypropylene and aluminum. U.S.Pat. No. 4,245,217, Passive Infrared Alarm System, issued Jan. 13, 1981to P. W. Steinhage which discloses an infrared filter having germaniumand dielectric layers. U.S. Pat. No. 4,284,323, Trichroic Mirror, issuedAug. 18, 1981 to G. Jankowitz which discloses a germanium and siliconmonoxide infrared filter on a quartz substrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an infrared filterthat can be manufactured relatively easily and inexpensively.

It is another object of the present invention to provide an infraredfilter that is composed of a plurality of laminated layers.

It is a further object of the present invention to provide an infraredfilter that is composed of a plurality of layers which may beindividually inspected prior to lamination thereof.

It is yet another object of the present invention to provide a methodfor manufacturing the filters and the individual layers of materialsthat make up the layers that is accurate and inexpensive.

The present invention includes a laminated structure that is composed ofa plurality of layers each having a uniform and relatively precisethickness. In a first embodiment relatively thick layers ofpolypropylene sheets are coated with a relatively thin layer ofgermanium utilizing a vacuum evaporation process. Internal layers ofpolypropylene are coated on both sides, whereas the upper and lowerexternal layers of polypropylene are coated only on their interior side.The interior surfaces of germanium are then joined together utilizing ahot melt adhesive, and the sandwiched layers are placed in a press witha suitable force to obtain proper lamination and adhesion. The devicethus contains a plurality of uniform layers which alternate inthickness. The thickness of the layers is selected to produce a devicewhich will act as an infrared filter in conjunction with the wavelengthsof infrared light with which the device will be utilized.

In an alternative embodiment uniform, relatively thick silicon wafersare alternately bonded with uniform, relatively thin polypropylenesheets to form a laminated structure that is suitable for use withdiffering wavelengths of infrared radiation.

Still another alternative embodiment may be formed utilizing siliconwafers of a uniform, relatively thick structure having uniform,relatively thin glass fibers disposed therebetween. An adhesive isintermixed with the glass fibers to bond the laminated structuretogether.

It is an advantage of the present invention that it provides an infraredfilter that can be manufactured relatively easily and inexpensively.

It is another advantage of the present invention that it provides aninfrared filter that is composed of a plurality of laminated layers.

It is a further advantage of the present invention that it provides aninfrared filter that is composed of a plurality of layers which may beindividually inspected prior to lamination thereof.

It is yet another advantage of the present invention that it provides amethod for manufacturing the filters and the individual layers ofmaterials that make up the layers that is accurate and inexpensive.

These and other objects, features and advantages of the presentinvention will become apparent from the following description of thepreferred embodiment and the accompanying drawings.

IN THE DRAWING

FIG. 1 is a perspective view of a preferred embodiment of the presentinvention;

FIG. 2 is a perspective view of an alternative embodiment of the presentinvention;

FIG. 3 is a perspective view of another preferred embodiment of thepresent invention;

FIG. 4 presents transmission data for a device such as that depicted inFIG. 1;

FIG. 5 presents transmission data for a device such as that depicted inFIG. 3;

FIG. 6 is a perspective view of a filter, such as is depicted in FIG. 3,having an internal anti-reflection coating; and

FIG. 7 is a laminated filter that is constructed as a band pass filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As depicted in FIG. 1, a first embodiment 10 of the present inventionincludes a laminated structure of several layers. Each layer is formedwith a rather uniform and precise thickness. The relatively thick layers12 in the first embodiment 10 depicted in FIG. 1 are composed of a lowindex of refraction material such as a polymer sheet composed ofmaterials such as polypropylene, polyethylene polyester, etc. Apreferred material is polypropylene, having a thickness of approximately20.0 microns, with a thickness variation of 0.3 microns. The interiorpolymer sheets 14 are coated on each surface with a film 16 composed ofa high index of refraction material, such as a germanium, silicon andgallium arsenide. The preferred embodiment includes a germanium film 16having a thickness of approximately 0.7 microns, with a thicknessvariation of 0.05 microns. The two external polymer layers 18 and 20have only an internal film coating 22 and 24 respectively. The variousfilm 16 coated polymer sheets are bonded together with an adhesive 30.In the preferred embodiment 10 the preferred adhesive is composed ofKodak A3 adhesive compound that is approximately 0.2 microns thick.Other adhesives such as paraffin can be utilized, however where thepresent invention is utilized in a cryogenic environment, the paraffinadhesive generally fails whereas the A3 adhesive generally remainsfunctional. In filters such as the first embodiment 10, the relativelythick polymer layers 12 have a low index of refraction as compared tothe relatively thin layers 16 that have a relatively high index ofrefraction. Such filters are useful in the infrared spectral range ofwavelengths from approximately 1.8 microns to approximately 1000microns, except in those wavelength bands where the layer material ishighly absorptive, such as approximately 7 microns and 14 microns forpolypropylene. The thickness of the layers 12 and 16 providedhereinabove are generally suited for an infrared filter that is designedfor a wavelength range of approximately 15 microns to 30 microns.

As will be understood by those skilled in the art, the relatively thicklayers 12 have a thickness corresponding to many quarter wavelengthoptical thicknesses of the incident radiation, whereas the thin layers16 have a thickness which corresponds to one quarter wavelength opticalthickness of the incident radiation. The close thickness tolerances ofthe alternating thick layers, as well as the thin layers, produces alaminated structure that has substantial dimensional regularitythroughout the overall thickness of the sandwich of layers.

FIG. 4 provides transmission data results for a laminated structure asdepicted in FIG. 1 composed of six polypropylene sheets having athickness of approximately 20.0 microns that are coated with thin filmsof germanium having a thickness of approximately 0.7 microns, and whichare bonded together using Kodak A3 adhesive having a thickness ofapproximately 0.2 microns. The filter 10 was sandwiched betweensubstrates of polypropylene having an index of refraction of 1.5 as iscustomarily done by those skilled in the art. The square waveform ofFIG. 4, having nearly equal bandwidths transmitted and reflected, andrelatively uniform periodicity is useful in many ways. To achieverelatively equal widths of the transmitted bands and the reflectedbands, a ratio of high index of refraction to low index of refraction ofthe first layers 12 to the second layers 16 respectively, equaling 7/3is preferred.

FIG. 2 depicts an alternative embodiment 110 of the present invention.As with the first embodiment 10, it includes a laminated structure ofseveral layers, each of which layers is formed with a rather uniform andprecise thickness. The relatively thick layers 112 are composed of arelatively high index of refraction material such as silicon, and in thepreferred embodiment are formed with a thickness of approximately 78.0microns, with a thickness variation of 0.1 microns. Each of the layers112 is separated by a second relatively thick layer 116 composed of amaterial having a relatively low index of refraction. In the preferredembodiment, the layers 116 are composed of polypropylene having athickness of approximately 20.0 microns, with a thickness variation of0.3 microns. An adhesive layer 118 composed of an adhesive such as KodakA3 is disposed between the first layers of silicon and the second layersof polypropylene to bond the structure together. In the preferredembodiment the adhesive layer 118 has a thickness of approximately 0.1microns. A filter of the type depicted in FIG. 2 is suitable forinfrared radiation in the spectral range of wavelengths fromapproximately 1.8 microns to 1000 microns. The particular embodiment ofFIG. 2 having layer thicknesses disclosed hereinabove is suitable forinfrared radiation in the wavelength range from approximately 15 micronsto 30 microns.

FIG. 3 depicts another alternative embodiment 210 of the laminatedinfrared filter of the present invention. It includes a plurality ofrelatively thick layers 212 that are preferably composed of siliconhaving a thickness of approximately 78.0 microns, with a thicknessvariation of 0.1 microns. The layers 212 are separated by glass fibers216 having a thickness of approximately 3.7 microns, with a thicknessvariation of 0.1 microns. The space 218 between the glass fibers 216 isfilled with Kodak A3 adhesive. The thickness of the second layers havingthe glass fibers 216 is approximately one quarter wavelength opticalthickness in the spectral range from 15 microns to 30 microns.

As would be obvious to one skilled in the art, other devices andmaterials could be used in place of the glass fiber spacers 216 depictedin FIG. 3. For instance, a coating of a material, such as alumina, couldbe deposited upon the surface of the silicon layer 212, using vacuumdeposition, chemical vapor deposition, sputtering or other suitablemeans, to achieve an appropriate thickness of approximately 3.7 microns.Thereafter, the alumina coating is masked and etched, such that aplurality of small areas having a diameter of approximately 3.7 micronsremain on the surface of the layer 212. The small areas then act asspacers in the same manner as the glass fibers 216. The dimensions ofthe small remaining areas should be less than one quarter wavelength ofthe infrared radiation, such that absorption and scattering are reducedto an acceptable level, such as approximately one percent per layer. Ofcourse, other types of spacer inserts could be utilized, and the presentinvention is not to be limited to the particular types of spacersdescribed hereinabove. As an alternative to spacers a layer of arelatively low index of refraction material, such as PbF₂ or ThF₄ may beutilized. The thickness of such a layer is approximately 3.7 microns,having a thickness variation of approximately 0.1 microns. A layer ofKodak A3 adhesive having a thickness of approximately 0.1 microns isthen used to bond the various layers together.

FIG. 5 provides transmission data results for a laminated structure asdepicted in FIG. 3 composed of six silicon layers having a thickness ofapproximately 78.0 microns that are separated by an adhesive layer ofKodak A3 adhesive and glass fibers having a thickness of approximately3.7 microns. The square waveform of FIG. 5, having nearly equalbandwidths passed and reflected, and relatively uniform periodicity isuseful in many ways. To achieve relatively equal widths of thetransmitted bands and the refracted bands a ratio of high index ofrefraction to low index of refraction equaling 7/3 is preferred.

It is possible to increase the effective bandwidth of the filters 10,110 and 210 beyond the one octave bandwidth depicted in FIGS. 4 and 5through the utilization of a plurality of anti-reflection coatingswithin the layers of the filter. Specifically, with regard to the filter10, an anti-reflection coating layer 302 may be formed on one surface ofthe polypropylene sheets 12 in the place of the coating thereof withgermanium. In the preferred embodiment, the anti-reflection coating ispreferably composed of selenium having a thickness of approximately onequarter wave optical thickness at a wavelength of 10 microns. Othermaterials such as diamond, cesium iodide and KRS 5 (a crystal compoundof thallium bromide and thallium chloride) can also be utilized.

The filter 210 may likewise include an anti-reflection coating 502formed on one inner surface of each silicon layer 212. Such a device isdepicted in FIG. 6, wherein similar layers are identified with identicalnumerals as utilized in FIG. 3. In the preferred embodiment, the filter510 is formed with anti-reflection coating 502 that is composed ofselenium, although diamond, cesium iodide and KRS 5 could be utilized,and is formed with a thickness approximating one quarter wavelengthoptical thickness at a wavelength of 10 microns. The thickness of thesecond layer having the adhesive compound 218 of A3 with glass rods 216or spacers would be reduced to 1.7 microns, which is one quarterwavelength optical thickness at a wavelength of 10 microns. Ananti-reflection coating 504 composed of a substance such aspolypropylene may also be disposed on the top face and bottom face ofthe filter 210, as is well known in the art.

A particular variation of the present invention is depicted in FIG. 7.The device of FIG. 7 is similar to a Fabry-Perot band pass filter, andit is similar in materials and construction to the filter 10 depicted inFIG. 1. As depicted in FIG. 7, the band pass filter 610 includes aplurality of first layers 612 composed of a uniform relatively thick,relatively low index of refraction material. The first layers 612 areseparated by a series of uniform relatively thin second layers 614composed of a relatively high index of refraction material. It is to benoted that two relatively thin layers 614 are disposed together to formcombined layer 616, which is therefore twice as thick as the layers 614.In the preferred embodiment, the layers 614 are formed by a depositionprocess on one surface of the layers 612, and a thin adhesive layer 618is disposed between the surface of the deposited thin layer 614 and thesurface of a subsequent first layer 612, such that the sandwichstructure 610 is bonded together with the adhesive layers 618. Anadhesive layer 620 is also disposed between the two relatively thinlayers 614 which comprise the combined layer 616. In the preferredembodiment, the relatively thick layers 612 are formed from a polymermaterial such as polypropylene, polyethylene or polyester (polypropylenebeing preferred) having a thickness of approximately 20 microns. Therelatively thin layers 614 are formed from a material such as germanium,silicon or gallium arsenide and have a thickness that corresponds to onequarter wavelength optical thickness at the filters tuned wavelength.Thus, for the preferred material of germanium, a thickness of 0.7microns corresponds to a one quarter wavelength optical thickness at atuned wavelength of 10 microns.

In discussing each of the preferred embodiments 10, 110 and 210, it hasbeen mentioned that the layers of material utilized in the filtersshould be of a relatively uniform thickness. The uniformity of thicknessis a significant factor in both the performance of individual filtersand the ability to inexpensively manufacture filters having uniform andrepeatable filtering characteristics. The relatively uniform thicknesspreferred to herein is generally of the order of 0.1 quarter wavelengthoptical thickness at the tuned wavelength of the particular filter,which, in the preferred embodiments described herein is 22 microns.

As is known to those skilled in the art, tolerances of the magnitude setforth hereinabove are not generally found in prefabricated materialssuch as silicon wafers. Thus, particular attention must be paid to theobtaining of materials having the relatively uniform thickness desired.

The fabrication of the first embodiment 10 is commenced by selecting asheet of polypropylene of low index of refraction, desired thickness anduniformity of optical thickness. The sheet is mounted in a vacuumevaporator and both sides are coated with a film of a high index ofrefraction material, such as germanium, to a desired thickness, such as0.7 microns thick. Other sheets are prepared having only one side coatedwith germanium. The coated sheet is then inspected, using its infraredspectral transmission to gauge absolute optical thickness, uniformity ofoptical thickness, and to detect coating absorption or thickness errors.A method for the inspection of the thickness of the coated sheets isdescribed hereinafter.

On a hotplate, heat a pair of optical flats, and a portion of adhesive,preferably Kodak A3, a hot melt adhesive made by Eastman Kodak. When theadhesive is molten, the fabrication is commenced by placing onto one ofthe flats, in the following sequence: a drop of paraffin, a sheet ofsingly coated polypropylene, coated side up, a drop of A3, a sheet ofdoubly coated polypropylene. Three cycles of the prior two items(adhesive and doubly coated polypropylene) are next performed, followedby a drop of A3, a singly coated polypropylene layer (coated side down),a drop of paraffin, and finally the second optical flat. Thismultilayered sandwich is then pressed together for approximately tenseconds using approximately five pounds of force to achieve minimaladherence between the layers.

The entire sandwich, including the optical flats, is then placed on a 30mm diameter by 2 mm thick teflon pad under a one ton press. Anotheridentical teflon pad is placed on top. Without delay (before the A3adhesive freezes), the combination is pressed together for thirtyseconds, using one thousand pounds of force to achieve lamination of thelayers.

After the lamination, the sandwich is allowed to cool for five minutes.Using a single edged razor blade, the combination is cleaved between theoptical flat and the first polypropylene sheet. Then using the razorblade at a very small angle to the optical flat's surface, the six-sheetcombination is cleaved between the remaining glass flat and the lastpolypropylene sheet. The resulting filter 10 is then cleaned withacetone using lens tissue or a Q-tip. The resulting laminated filter 10may be used free standing or sandwiched between substrates. FIG. 1 showsthe freestanding lamination.

As for the devices 110 and 210, silicon wafers are fabricated to thelayer thickness desired utilizing a grinding and polishing procedurethat includes the mounting of relatively thick silicon wafers on coatedblocking plates. To produce coated blocking plates, grind and polishfused quartz blanks 150 mm diameter by 50 mm thick. The back surface maybe a commercial polish, but the front surface should be flat to 0.25wavelength at 0.546 micron. The front surface is then coated by vacuumevaporation, sputtering or some other appropriate means to have aboutforty percent reflection in the visible. A trilayer of zirconia,alumina, and zirconia was used, wherein each layer was one quarterwavelength optical thickness at 0.55 micron. Zirconia was chosen for itsmechanical durability as was alumina. The goal is about 40 percentreflection and durability to withstand several cycles of the blockingprocess.

To fabricate silicon discs as thin as 60 microns, flat and parallel to0.1 micron, coated quartz blocking plates as described above are used.Obtain or fabricate, using standard optical procedures, single crystalsilicon discs ground and polished flat to one quarter wavelength at0.546 micron. Their thickness should be at least one eighth of theirdiameter Block these discs using color contact onto a blocking plate.The procedure described below, although not necessary, was designed toease this blocking process.

To color contact silicon discs, 25 mm diameter by 4 mm thick, onto a 150mm diameter by 25 mm thick fused quartz blocking plate, start withsilicon discs as described above and a coated blocking plate asdescribed above. Put the blocking plate coated side up on a hot plate.Heat it until paraffin melts on its surface. Flood the surface withmolten paraffin. Illuminate the bottom surface with a low pressuremercury lamp filtered to 0.546 micron.

Place a silicon disc on the molten paraffin and press on it until greenfringes are seen from underneath. These are formed between the siliconsurface and the coating on the blocking plate. They are very highcontrast and easy to see because the coating's reflection is comparableto silicon's. To facilitate this observation, put 0.546 micron sourceadjacent to the hot plate, facing upward. Elevate the blocking plateabout 100 mm above the hot plate, supporting it in a metal frame or on aglass plate. Put a mirror facing upward adjacent to the hot plate,opposite the spectral lamp. With this setup, you may look into themirror and see the fringes formed under the silicon discs.

Further pressing broadens the green fringes and finally produces coloredfringes which can be seen in fluorescent light from a typical lightfixture. Once all the discs (typically 19, 37 or 61) are blocked in thismanner, they are further pressed, to broaden the colored fringe and makeit uniform. Allow the paraffin to freeze and check that the undersidestill displays broad colored fringes under fluorescent lightillumination. It has been useful to clean the paraffin from between thesilicon discs using a tissue dampened with acetone, then painting achamfer of lacquer around each silicon disc. The lacquer protects theparaffin and the edge of the wafer from undue attack by the grinding andpolishing compounds.

The color-contacted block of silicon discs is ground and polished usingconventional optical procedures to a thickness of 60+-0.1 microns. Thisparallelism, while not common in the optical field, is done using wellknown procedures. This precision in absolute thickness is even lesscommon. A micrometer measurement gets you to 61+-0.5 microns, thenspectral measurement of one disc carefully removed from the block,indicates its thickness to 0.1 micron. For example, if the discs arestill too thick by one half micron, then the removal of 0.5/0.546wavelengths of material at 0.546 micron is conducted. This translates toremoving three fringes, which is measurable using standardinterferometric procedures.

In a preferred embodiment to determine the precise absolute thickness ofa silicon wafer, one measures its infrared transmission between 16 and34 microns. This spectrum should be a normal interference pattern withmany peaks. Choose a peak near 16 microns and designate the wavelengthof that peak λ_(o). Upon inspecting the region near 2λ_(o), either apeak or a valley will exist. Not counting the valley very near 2λ_(o),count the number of valleys between λ_(o) and 2λ_(o), and call thisnumber p. Now write down the indicated optical thickness which is pλ_(o) if 2λ_(o) exhibits a peak. If the transmission at 2λ_(o) is avalley, the indicated optical thickness is (p+0.5) λ_(o). The physicalthickness can then be calculated by multiplying the optical thickness bythe index of refraction of silicon at λ_(o). The measurement of thethickness of the polymer sheets utilized in various embodimentsdisclosed herein can also be accomplished utilizing this procedure.

The filter 110 is fabricated utilizing the silicon layers producedutilizing the method above-described. The wafers 112 are alternated witha layer 118 of A3 Kodak adhesive, a polypropylene sheet 116 and anotherlayer 118 of A3 adhesive. The sandwich device is then placed in a oneton press for approximately 30 seconds to create the laminated filter110.

The filter embodiment 212 includes silicon layers laminated withadhesive layers of controlled thicknesses. The thickness of the adhesivelayer is controlled by spacers such as the glass fibers 216 aluminadeposits, or a layer of PbF₂ or ThF₄, as discussed above. To produce anadhesive containing glass rod spacers, mix 10 milligrams of glass rods(3+-0.1 micron diameter by 10+-1 micron long, obtained from NipponElectric Glass Co. (TM)) into one gram of molten Kodak (TM) A3 adhesive.Stir until it looks uniformly turbid. The resulting solution is lessthan one half of one percent by volume glass. When pressed betweenoptically smooth and flat surfaces, it will stop at a thickness of threemicrons, at which time glass will obscure less than one percent of thesurface area. Transmission loss due to absorption and scattering by theglass spacers is thus less than one percent per spaced adhesive layer.

The lamination process of the present invention allows individual layersto be inspected and qualified prior to lamination. This process, unlikevacuum deposition, gets easier as the wavelength of the infraredradiation gets longer because the individual layers become thicker andeasier to handle. This process allows inexpensive lamination of coatedsheets to produce a filter, as compared to the expensive, time consumingand low output processes which currently require vacuum evaporation anddeposition processes to produce the relatively thick layers necessaryfor an infrared filter.

While the invention has been particularly shown and described withreference to certain preferred embodiments, it will be understood bythose skilled in the art that various alterations and modifications inform and detail may be made therein. Accordingly, it is intended thatthe following claims cover all such alterations and modifications as mayfall within the true spirit and scope of the invention.

What I claim is:
 1. An infrared filter comprising:a plurality ofsubstantially identical first layers; a plurality of substantiallyidentical second layers; said first layers and said second layers beinginterleaved, such that at least one said second layer is disposedbetween each said first layer; each said first layer and each saidsecond layer being substantially planar and having a substantiallyuniform physical thickness; the material comprising said first layershaving a different refractive index from the material comprising saidsecond layers; said first layers having a different optical thicknessfrom said second layers; said material comprising said first layershaving a higher index of refraction than said material comprising saidsecond layers; said material comprising said first layers being silicon,and said material comprising said second layers being an adhesivecompound; wherein a plurality of spacers are disposed within each saidsecond layers to separate said first layers; and wherein each of saidsecond layers has a thickness of approximately one quarter wavelengthoptical thickness at a particular tuned wavelength and said thickness ofsaid first layers is at least a plurality of quarter wavelength opticalthicknesses.
 2. An infrared filter comprising:a plurality ofsubstantially identical first layers; a plurality of substantiallyidentical second layers; said first layers and said second layers beinginterleaved, such that at least one said second layer is disposedbetween each said first layer; each said first layer and each saidsecond layer being substantially planar and having a substantiallyuniform physical thickness; the material comprising said first layershaving a different refractive index from the material comprising saidsecond layers; said first layers having a different optical thicknessfrom said second layers; said material comprising said first layershaving a lower index of refraction than said material comprising saidsecond layers; said material comprising said first layers beingpolypropylene, and said material comprising said second layers beinggermanium; andwherein said polypropylene first layers are formed with athickness of approximately 20 microns, and wherein two said germaniumsecond layers are disposed between each of said first layers, andwherein each said germanium layer is formed with a thickness ofapproximately 0.7 microns; and wherein a layer of adhesive compound isdisposed between said two germanium second layers to bond said layerstogether.
 3. An infrared filter comprising:a plurality of substantiallyidentical first layers; a plurality of substantially identical secondlayers; said first layers and said second layers being interleaved, suchthat at least one said second layer is disposed between each said firstlayer; each said first layer and each said second layer beingsubstantially planar and having a substantially uniform physicalthickness; the material comprising said first layers having a differentrefractive index from the material comprising said second layers; saidfirst layers having a different optical thickness from said secondlayers; andwherein at least one interior surface of said first layers iscontacted with an anti-reflection coating.
 4. An infrared filter asdescribed in claim 3 wherein the thickness of said anti-reflectivecoating is one quarter wavelength optical thickness at a particulartuned wavelength.
 5. An infrared filter as described in claim 3 whereinsaid anti-reflection coating is composed of a material from the groupconsisting of selenium, diamond, cesium iodide and KRS
 5. 6. An infraredfilter comprising:a plurality of substantially identical first layers; aplurality of substantially identical second layers; said first layersand said second layers being interleaved, such that at least one saidsecond layer is disposed between each said first layer; each said firstlayer and each said second layer being substantially planar and having asubstantially uniform physical thickness; the material comprising saidfirst layers having a different refractive index from the materialcomprising said second layers; said first layers having a differentoptical thickness from said second layers; andwherein the thickness ofeach said second layer is approximately one quarter wavelength opticalthickness at a particular tuned wavelength, and the thickness of eachsaid first layer is at least a plurality of quarter wavelength opticalthicknesses.
 7. An infrared filter comprising:a plurality ofsubstantially identical first layers; a plurality of substantiallyidentical second layers; said first layers and said second layers beinginterleaved, such that at least one said second layer is disposedbetween each said first layer; each said first layer and each saidsecond layer being substantially planar and having a substantiallyuniform physical thickness; the material comprising said first layershaving a different refractive index from the material comprising saidsecond layers; said first layers having a different optical thicknessfrom said second layers;wherein the thickness of at least one of saidsecond layers disposed between said first layers is approximately twoquarter wavelength optical thicknesses at a particular tuned wavelength,and the thickness of others of said second layers is approximately onequarter wavelength optical thickness, and the thickness of each saidfirst layers is at least a plurality of quarter wavelength opticalthicknesses.
 8. A method for manufacturing an infrared filter comprisingthe steps of:fabricating a plurality of first layers formed from wafershaving a substantially uniform thickness; placing a plurality of secondlayers between said first layers in an interleaved sandwich structure;applying an adhesive between said first layers and said second layerswithin said sandwich structure; applying pressure to said sandwichstructure to create a laminated filter; said wafers being formedutilizing a grinding and polishing process to achieve a uniform,substantially identical thickness for each of said wafers; said waferscomprising said first layers being composed of a material having arelatively high index of refraction and wherein said second layers arecomposed of a material having a relatively low index of refraction;andwherein said first layers are composed of silicon and said secondlayers are composed of an adhesive compound, and wherein spacers aredisposed within each said second layer to separate said first layers. 9.A method for manufacturing an infrared filter as described in claim 8wherein said spacers are composed of glass fibers.
 10. A method formanufacturing an infrared filter as described in claim 8 wherein saidspacers are composed of small islands of solid material.
 11. A methodfor manufacturing an infrared filter as described in claim 8 whereinsaid spacers are formed from a coating of a relatively low index ofrefraction material disposed upon an interior surface of each saidwafer.
 12. A method for manufacturing an infrared filter as described inclaim 11 wherein said coatings are composed of either PbF₂ or ThF₄.